Method for the uniform electroplating of sheet and strip

ABSTRACT

In the continuous electroplating of sheet and strip, heavy edge build up is avoided by the use of a new anode design. Proper anode design is achieved by (1) determining the coating weight profile produced by a rectangular anode of full pass width and (2) shaping the anode by tapering a portion of the sides thereof to a width approximately equal to the predetermined width of uniform plating.

United States Patent Hoeckelman Dec. 17, 1974 METHOD FOR THE UNIFORM ELECTROPLATING OF SHEET AND STRIP [75] Inventor: Ralph F. l-loeckelman, lrwin Borough, Pa.

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

22 Filed: June 13, 1973 21 App1.No.: 369,644

9/1954 Hassell 204/206 OTHER PUBLICATIONS l-larriss, IBM Technical Disclosure Bulletin, Vol. 6, No. 8, Jan. 1964, Segmented Electroplating Contact Roll, p. 68.

Primary Examiner-John H. Mack Assistant Examiner-Wayne A. Langel Attorney, Agent, or Firm-Arthur .l. Greif [5 7] ABSTRACT In the continuous electroplating of sheet and strip, heavy edge build up is avoided by the use of a new anode design. Proper anode design is achieved by (l) determining the coating weight profile produced by a rectangular anode of full pass width and (2) shaping the anode by tapering a portion of the sides thereof to a width approximately equal to the predetermined width of uniform plating.

7 Claims, 8 Drawing Figures c, METAL mg/fr 5; n Q Q l l I O 5 l l STRIP W/DTH- INCHES PATENTEUBEBI m 3,855,083

. sum 3 or 4 l I I /0 I5 20 STRIP W/D TH-INCHES F l6: 3

METHOD FOR THE UNIFORM ELECIROPLATING OF SHEET AND STRIP This invention relates to a method for the continuous electroplating of elongated flat rolled articles such as sheet and strip and is more particularly directed to a method for preventing heavy coating build up on the edges of such articles. The term strip will hereinafter be employed to define such flat rolled articles.

In the continuous electroplating of strip, the flat surfaces thereof are passed in parallel relation to one or more generally flat anodes immersed in an electroplating electrolyte. The anodes are generally of a width about equal to the width of the strip being plated. In the use of such equal width anodes, problems are often encountered in the achievement of uniform plating. One source of such problems is mechanical in nature and is a result of poor strip tracking, i.e. wandering of the strip from the center of the pass line. Such wandering may cause one edge of the strip to receive a coating heavier than desired, while the other edge receives little or no coating. A second major source of such problems is electrochemical in nature, and is a result of the concentration of the plating current at the edges of the strip. This edge effect is caused by the tendency of current density lines to be attracted to a sharp point or series of points, and is especially pronounced in plating systems with poor throwing power. The uneven plating due to poor strip tracking has, in most instances, been overcome by employing anodes of sufficient width to cover the extreme cases of strip wandering. A number of solutions have been proposed to overcome the electrochemical problem. One, such proposal employed anodes which were elliptical in cross-section. This technique is based on the principle that current density increases as the distance between electrodes decreases. Thus, the elliptical anode provides a shorter distance between the anode at the center of the strip; increasing the current density in that area. However, this proposal has generally proved unreliable and cumbersome since it does not overcome strip tracking problems and the anode itself is difficult to both manufacture and install. Another technique for overcoming edge effect is the utilization of a mask or shielding device which is deposed between the anode and the strip, at the outer edges thereof. While such masks are effective in reducing the current density at the edges, they require the use of-intricate mechanical and/or electrical devices to automatically adjust to the wandering strip. Equally important, they must generally be limited to plating processes wherein the anode is some distance from the strip. In processes wherein the anode to cathode distance is small (e.g. 1-2 inches) rough edges or torn strip could hook on to the mask and either cobble the strip or destroy the mask. Finally, attractor wires, adjacent the strip edges, have been employed to divert a Other objects and advantages of the instant invention" will be more apparent from a reading of the following description when taken in conjunction with the appended claims and the drawings, in which:

FIG. 1 is a coating weight profile obtained from a strip coated with Cr and employing conventional rectangular anodes;

FIGS. 2(a) and 2(b) describe a preferred embodiment for designing the anode of this invention;

FIG. 3 is a coating weight profile obtained from a strip coated with Cr and employing the anode design of FIG. 2;

FIGS. 4a-d are representations of altemativeanode designs useful in the instant invention.

It has now been found that strip can be electroplated, with a degree of unifomiity equal to or superior to that of the above mentioned prior art techniques, by employing an anode configuration which regulates the time of metal deposition for different parts of the strip. This is in contrast to the above techniques which regulate the current density at different areas of the strip. Basically the new configuration employs a tandemarray of two anode fractions, wherein the face of one fraction is generally rectangular (or square) and the face of the second fraction is generally trapezoidal. In a preferred embodiment, the desired configuration is a single anode comprising the two requisite tandem fractions.

In the design of the proper anode dimensions, it is first necessary to determine the width of uniform plating which may be achieved using a conventional rectangular anode forrn, the width of which, Wa, is approximately equal to the width of the strip to be plated. Preferably, the width, Wa, will be ofa dimension suffistrip. The anode will be of conventional length, and is prescribed by the line speed, current density, number of anodes in series, etc., which are to be employed in the particular plating process. The determination of the proper length, L, is well known to those skilled in the art. A coating profile is then made describing the variation in coating metal thickness along the entire width of the strip. From a plot of this profile, the points at which the coating thickness abruptly increases, i.e. ,the points of inflection of the curve, are then determined. The distance between these points is defined as the width of uniform coating. In utilizing the preferred embodiment of a single anode, a line of length Wb, approximately equal to said width of uniform plating is then laid out and centered on either the top or bottom edge of the anode. Lines are then drawn from a point (e.g. the rnidlength point) along the sides of the anode to connect with the end points of the previously laidout line, Wb. The portion of the anode proscribed by these lines is then cut away.

The above embodiment was applied to the design of an anode for the electroplating of a steel strip with a principally chromium coating, using a method similar to that described in U.S. Pat. No. 3,642,587, the disclosure of which is incorporated herein by reference. Steel strip 35% inches wide was passed through a chromium plating bath at a speed of approximately 1,500 ft./min. Full pass width rectangular anodes, 38 inches wide and 22% inches long were employed in the initial plating operation for determination of the coating weight profile (FIG. 1). A visual approximation of the inflection points, shows that about 23 inches of the center section has what may be temted a unifonn coating weight, i.e. Wb==23 inches. With reference to FIG. 20, W12 was then laid out and centered on the bottom edge of the anode, and a line drawn from the end points of Wb to each of the midlength points l 1 Mt inches from the bottom of the anode) of the anode side. The anode was then cut along these lines to provide the configuration of FIG. 2b. This latter anode configuration (FIG. 2b) was then installed for further electroplating, using substantially the same parameters as were employed with the conventional rectangular anode form. The coating weight profile of the resultant product is shown in FIG. 3. The coated strip exhibited a uniform, bright appearance. The obtention of such uniform appearance is particularly complicated in the production of the above Cr plated objects, since such coatings are generally composed of a very thin chromium metal layer and an overlying chromium oxide layer with a thickness of the order of about 1 mg./ft. (-0.0l microns). If any portion (i.e. the edges) of the oxide layer is appreciably thicker than about 2.0 mg./ft. that portion will be brownish in color; an appearance which is undesirable for most TFS applications. On the other hand, if the oxide layer at the edges is thin enough to provide a desired bright-appearance, both the metallic underlayer and the overlying oxide layer at the central portion of the strip may be too thin to provide desired corrosion protection and lacquerability.

In view of the simplicity of its design, the anode employed in illustrative example above is a most preferred embodiment of the instant invention. It should be clear, however, that various design modifications will provide substantially equivalent plating uniformity. The essential feature of all such design modifications is that they provide a decreased time of electrodeposition for the edge portions of the strip exhibiting undesirably high coating thickness. FIGS. 4a through 4d are illustrative of some of the many modifications which may be employed without departing from the scope of the present invention.

FIG. 4a is illustrative of the fact that it is not necessary that the rectangular fraction and the trapezoidal fraction be of equal length. Thus, the length of the rectangular fraction may vary from about L/3 to 2L/3. In such a case the length of the trapezoidal fraction will then correspond to such length and vary respectively from about 2L/3 to L/3. FIG. 4b incorporates a further modification, wherein the total desired length L is provided by separate anode fractions tandemly aligned with respect to the strip pass line. This figure also shows that the generally trapezoidal fraction of the anode may also be placed on top of rectangular fraction. FIG. 4c shows that the sides of the fractions may depart somewhat from linearity, while FIG. 4d shows that the trapezoidal fraction itself (or the rectangular fraction as well) be composed of more than one sub-fraction. Fi-

nally, it may readily be seen that it is not necessary that the shorter anode edge Wb be equal to the predetermined width of uniform plating. Thus, undesirable edge effects can substantially be eliminated if Wb is approximately equal to such uniform plating width. However, it is preferable that Wb not vary by more than about 10 percent from such uniform plating width.

I claim: I. In the method for the continuous electroplating of a principally metallic coating onto a metal strip, which includes a. through a plating electrolyte, passing the strip in parallel relation to a generally rectangular face of an anode form having a length L and a width Wa, wherein Wa is approximately equal to the width of said strip, and b. applying a current density between said anode form and said strip, at a magnitude and for a time, sufiicient to effect the deposition of a substantially uniform desired coating weight along a width of said strip, said coating exhibiting an edge effect,

the improvement for substantially eliminating said edge effect, which comprises employing an anode configuration composed of two tandemly aligned fractions: the first fraction having a face which is generally rectangular, said first fraction face having a width, Wu, and a length varying from about US to 2L/3; the second fraction having a face which is generally trapezoidal, said second fraction face having a corresponding length of 2L/3 to L/3 respectively, and a width tapering approximately linearly and decreasing from a width Wa to a width Wb, wherein Wb is approximately equal to the predetermined width of uniform coating weight produced by said rectangular anode form.

2. The method of claim 1,.in which said anode configuration comprises a single anode, and wherein an end of said first fraction having a width Wa, is coincident with the end of said second fraction having a width Wu.

3. The method of claim 2, wherein Wa is of a dimension sufficient to accommodate variations in the pass line of said strip and Wb is from about 0.9 to 1.1 the dimension of said predetermined uniform width.

4. The method of claim 3, wherein the surface of said strip is passed between the substantially parallel faces of two such anode configurations, whereby the coating metal is electroplated on both surfaces of said strip.

5. The method of claim 4, wherein said strip metal is steel and the metal in said coating is chromium.

6. The method of claim 5, wherein said anode configurations are positioned substantially vertically in said electrolyte.

7. The method of claim 6, wherein the lengths of said first and second fractions are substantially equal. 

1. IN THE METHOD FOR THE CONTINUOUS ELECTROPLATING OF A PRINICIPALLY METALLIC COATING ONTO A METAL STRIP, WHICH INCLUDES A. THROUGH A PLATING ELECTROLYTE, PASSING THE STRIP IN PARALLEL RELATION TO A GENERALLY RECTANGULAR FACE OF AN ANODE FROM HAVING A LENGTH L AND A WITH WA, WHEREIN WA IS APPROXIMATELY EQUAL TO THE WIDTH OF SAID STRIP, AND B. APPLYING A CURRENT DENSITY BETWEEN SAID ANODE FROM AND SAID STRIP, AT A MAGNITUDE AND FOR A TIME, SUFFICIENT TO EFFECT THE DEPOSITION OF A SUBSTANTIALLY UNIFORM DESIRED COATING WEIGHT ALONG A WIDTH OF SAID STRIP, SAID COATING EXHIBITING AN EDGE EFFECT, THE IMPROVEMENT FOR SUBSTANTIALLY ELIMINATING SAID EDGE EFFECT, WHICH COMPRISES EMPLOYING AN ANODE CONFIGURATION COMPOSED OF TWO TANDEMLY ALIGNED FRACTIONS: THE FIRST FRACTION HAVING A FIRST WHICH IS GENERALLY RECTANGULAR, SAID FIRST FRACTION FACE HAVING A WIDTH, WA, AMD A LENGTH VARYING FROM ABOUT L/3 TO 2L/3, THE SECOND FRACTION HAVING A FACE WHICH IS GENERALLY TRAPEZOIDAL, SAID SECOND FRACTION FACE HAVING A CORRESPONDING LENGTH OF 2L/3 TO L/3 RESPECTIVELY, AND A WIDTH TAPERING APPROXIMATELY LINEARLY AND DECREASING FROM A WIDTH WA TO A WIDTH WB, WHEREIN WB IS APPROXIMATELY EQUAL TO THE PREDETERMINED WIDTH OF UNIFORM COATING WEIGHT PRODUCED BY SAID RECTANGULAR ANODE FORM.
 2. The method of claim 1, in which said anode configuration comprises a single anode, and wherein an end of said first fraction having a width Wa, is coincident with the end of said second fraction having a width Wa.
 3. The method of claim 2, wherein Wa is of a dimension sufficient to accommodate variations in the pass line of said strip and Wb is from about 0.9 to 1.1 the dimension of said predetermined uniform width.
 4. The method of claim 3, wherein the surface of said strip is passed between the substantially parallel faces of two such anode configurations, whereby the coating metal is electroplated on both surfaces of said strip.
 5. The method of claim 4, wherein said strip metal is steel and the metal in said coating is chromium.
 6. The method of claim 5, wherein said anode configurations are positioned substantially vertically in said electrolyte.
 7. The method of claim 6, wherein the lengths of said first and second fractions are substantially equal. 